1. Field of the Invention
This invention relates to a hydrogenated amorphous silicon (hereinafter referred to as "a-Si:H") photoconductive layer sensitive to electromagnetic wave such as light (this light broadly means ultraviolet ray, visible ray, infrared ray, X-ray, gamma ray and the like).
2. Description of the Prior Art
Since the structure of an amorphous silicon (hereinafter referred to as "a-Si") layer varies depending upon the type of the process for production and the conditions under which the production is carried out, the resulting electric, optical and photoelectric characteristics also change. Therefore, the reproducibility is very poor.
On the other hand, an a-Si layer composed of silicon only tends to have dangling bonds, vacancies voids and the like on the surface and in the inside which are formed due to a particular structure of an amorphous material, and there are formed energy states due to them. As the result, it is very difficult to form a layer which is of a high reproducibility of electric, optical and photoelectric characteristics.
In particular, such dangling bonds exist inevitably, and therefore, localized states are formed in the band gap as, so-called, gap states.
Therefore, when the a-Si is used, for example, as a photoconductive layer for a solid state imaging device and an image forming member for electrophotography, the photoconductive layer is not always satisfactory with respect to photosensitivity, photo-responsing property, and SN ratio [photo-current (ip)/dark current (id)].
If a-Si layers having fairly few gap states can be formed, the above mentioned problems can be solved and in addition, it becomes possible to introduce impurities and thereby form a desired energy level in the band gap depending upon the type of the impurity. As the result, a-Si layers having desired characteristics and high reproducibility can be produced.
Heretofore, it has been attempted that hydrogen is introduced into the a-Si layer upon forming the layer to eliminate the dangling bonds for the purpose of diminishing the influence of the dangling bonds.
However, at the present time, a simple introduction of hydrogen into an a-Si layer does not succeed in controlling the characteristics and reproducing.
This failure seems to be attributable to the fact that the hydrogen thus introduced is present in the a-Si layer in various forms.
Assuming that hydrogen atom is chemically bonded to silicon atom, they can be in the three types, that is, SiH, SiH.sub.2 and SiH.sub.3, and further even if hydrogen atom is not chemically bonded to silicon atom, it is considered that electrons contributing to the bond are biased to one of Si atom and H atom and are in an ionized state, or hydrogen is taken in the voids of the layer in a form of H.sub.2 or H and adsorbed thereto, and further it is surely considered that the existence state of the hydrogen affects the energy states of a-Si layer.
Therefore, from the point of view of application of a-Si in future as well as studying fundamental physical properties, it is very desirable to study what state in which the hydrogen is taken in is suitable for obtaining an a-Si layer having desired electric, optical and photoelectric characteristics and for producing the a-Si layer with a high reproducibility.
In particular, if such a-Si layer is developed, the a-Si layer can be used for solar cells, photoconductive layers equipped to image forming apparatuses, and photoelectric transducer layers for reading apparatuses with satisfactory electric, optical and photoelectric characteristics, and moreover, the a-Si layer having a large surface area can be produced. This is one of the features of amorphous silicon. In view of the foregoing, development of such a-Si layer contributes to industry to a great extent.